1. Field of the Invention
This invention relates to an electric power supply device for feeding electric power from a plurality of power sources of different kinds such as a power line and dry cells or secondary batteries, to an electronic apparatus.
2. Description of the Related Art
The electric power supply devices of the kind capable of selectively feeding electric power from any of a plurality of power sources of different kinds such as a power line and dry cells or secondary batteries to an electronic apparatus which is operated by the supply of electric power have been arranged as shown in FIGS. 1 and 2.
FIG. 1 is an oblique view showing the conventional electric power supply device. FIG. 2 shows a circuit equivalent to the device shown in FIG. 1.
Referring to FIGS. 1 and 2, a battery pack 41 is arranged to contain dry cells or secondary batteries as a power source. A power supply plug 42 is arranged to supply DC power from an AC/DC (AC-to-DC) converter or the like. A power supply jack 43 includes a change-over switch 43b which is arranged to prevent duplex power feeding by giving priority to the selection of the power supplied through the power supply plug 42 when the power supply plug 42 is inserted into the power supply jack 43. Battery pack contacting contacts 44 are disposed within the body of an electronic apparatus and arranged to come into contact with the contacts of the battery pack 41. A voltage transforming circuit unit 45 is arranged to perform a voltage transforming action on the supplied electric power into a power suited for each of applicable electric circuits and to distribute and feed the voltage-transformed power to each electric circuit. Reference numerals 46a to 46d respectively denote power transmitting lead wires.
In the arrangement shown in FIGS. 1 and 2, the power outputted from the battery pack 41 (i.e., an input current) comes from the battery pack contacting contacts 44 and, after that, is supplied to the voltage transforming circuit unit 45 through the lead wire 46a, the change-over switch 43b disposed within the power supply jack 43 and the lead wire 46c. A return current flows into the battery pack 41 through the lead wire 46b and the battery pack contacting contacts 44.
In a case where DC power is to be supplied to the electronic apparatus from an AC/DC converter or the like (not shown) via the power supply plug 42, the position of the change-over switch 43b within the power supply jack 43 is changed by inserting the power supply plug 42 into the power supply jack 43. The power supplied via the power supply plug 42 and the power supply jack 43 is then supplied to the voltage transforming circuit unit 45 via the lead wire 46c. A return current then flows via the lead wire 46d to a power supply plug contacting terminal 43a disposed within the power supply jack 43.
FIGS. 3 and 4 show another arrangement example of the conventional electric power supply device. FIG. 3 is an oblique view of the device while FIG. 4 shows a circuit equivalent to the device shown in FIG. 3. In the arrangement shown in FIGS. 3 and 4, all component parts arranged in the same manner as those of FIGS. 1 and 2 are indicated by the same reference numerals and the details of them are omitted from the following description.
Referring to FIGS. 3 and 4, lead wires 46e and 46f are provided for power transmission between the battery pack contacting contacts 44 and the voltage transforming circuit unit 45. A dummy battery pack 47 is arranged in about the same shape as the battery pack 41 and to has power supply jack 43 and contacts 47a.
With the conventional electric power supply device arranged as shown in FIGS. 3 and 4, the provision of the dummy battery pack 47 enables the power supplied from the power supply plug 42 to be supplied via the power supply jack 43 and the contacts 47a to the battery pack contacting contacts 44 disposed within the electronic apparatus in the same manner as the battery pack 41 which is shown in FIGS. 1 and 2.
The conventional electric power supply device shown in FIGS. 1 and 2 is capable of preventing the so-called duplex power feeding, i.e., simultaneous supply of power from different kinds of power sources. However, since the power supplied from the battery pack contacting contacts 44 is supplied via the power supply jack 43, the power transmission path includes the lead wire 46a, the change-over switch 43b and the lead wire 46c, and the adverse effect of the impedance resulting from the lengths of these lead wires, the contact resistance of the change-over switch 43b, etc., causes an extremely high impedance of the power supply path from the battery pack contacting contacts 44 to the voltage transforming circuit unit 45.
Therefore, in a case where the current level of the power supplied from the battery pack 41 is high, a voltage drop would occur. Under such a condition, the electronic apparatus tends to be shut down before the power of the battery pack 41 is sufficiently used up.
In the case of the conventional electric power supply device shown in FIGS. 3 and 4, the impedance of the power supply path from the battery pack contacting contacts 44 to the voltage transforming circuit unit 45 can be lowered. However, this electric power supply device necessitates the use of the dummy battery pack 47, which increases the size of arrangement of the apparatus and thus results in an increase in cost. Further, in a case where power is to be supplied from the AC/DC converter or the like through the power supply plug 42, if the battery pack 41 has already been loaded into the electric power supply device, the battery pack 41 must be replaced with the dummy battery pack 47. Such a replacing operation undesirably degrades operability.